Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), astigmatism, and defects in near range vision usually associated with aging (presbyopia).
It is believed that presbyopia occurs as a person ages when the lens of the eye begins to crystallize and lose its elasticity, eventually resulting in the eye losing the ability to focus at near distances, such as the normal reading distance, and in some cases at intermediate distances. Presbyopic persons (presbyopes) complain of difficulty performing close tasks. To compensate for presbyopia, ophthalmic lenses are required that are more positively powered or less negatively powered than the distance correction. Some presbyopic persons have both near vision and far vision defects, requiring segmented bifocal or multifocal lenses or progressive multifocal lenses, instead of single vision lenses, to properly correct their vision.
Astigmatism occurs as the refractive error in an eye is dependent upon meridian. This is usually due to one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape. It may also be due to one or more surfaces being transversely displaced or tilted. Astigmatism is usually regular, which means that the principal (maximum and minimum power) meridians are perpendicular to each other. People with astigmatism have blurred vision at all distances, although this may be worse at distance or near, depending on the type of astigmatism. These people may complain of sore eyes and headaches associated with demanding visual tasks. Astigmatism can corrected with an astigmatic ophthalmic lens, which usually has one spherical surface and one toroidal (cylindrical) surface.
Large population of presbyopes also has an astigmatism vision error. Those presbyopes may require to wear contact lenses capable of correcting both astigmatism and presbyopia. Such contact lenses are disclosed in a co-pending U.S. patent application Ser. No., entitled Toric Multifocal Contact lenses, and have a cylindrical optical surface (or power) to correct astigmatism vision errors and a multifocal power to compensate for presbyopia.
Effective use of a contact lens, such as a toric contact lens, a segmented multifocal contact lens or a toric multifocal contact lens, requires a predetermined orientation on an eye. One of the most commonly utilized basic techniques, which can maintain a predetermined orientation of a contact lens on an eye, is prism stabilization. By using this technique, a contact lens is provided with prism ballast, which is generally a base-down prism to increase the mass of the lower portion of the lens and to create a weighting effect to orient the lens. However, there are some disadvantages associated with designs of contact lenses having prism ballast in the prior art. Those lenses are not comfortable to wear and/or not highly effective in maintaining lens orientation. Furthermore, current contact lenses with prism ballast as orientation feature may not be able to provide better visual performance since they tend to have optical distortions known as prism.
Therefore, there is a need for a contact lens with an orientation feature, wherein the lens is substantially free of prism optical effects and is comfortable to wear.
An object of the invention is to provide a contact lens having an orientation feature which can maintain a predetermined orientation on an eye while it does not cause prism optical distortions.
Another object of the invention is to provide a family of toric and/or multifocal contact lenses each having an orientation feature which can maintain a predetermined orientation on an eye while it does not cause prism optical distortions.
A further object of the invention is to provide a method for producing a contact lens having an orientation feature which can maintain a predetermined orientation on an eye while it does not cause prism optical distortions.